1. Field of the Invention
The present invention pertains to apparatus and methods for detecting the presence or absence of material at a predetermined location. More particularly, the present invention relates to systems for determining the amount of material accummulated at a locale. Apparatus and methods of the present invention find particulr application in the case of material containers, such as bins, wherein the amount of material present may be determined by detecting the level of material. Appropriate indicators, or warning devices, may be triggered ultimately by use of the present invention, and systems for adding material, or removing material, may also be operated in response to detection of the material level by use of the present invention.
2. Description of the Prior Art
Several prior art systems available for measuring material level in bins include the use of mechanical probes which extend into the bin. Such a probe is made to vibrate, and the physical characteristics of the elastic disturbances, which are affected by the quantity of material present about the probe, are analyzed in terms of the quantity of material present.
Other prior art designs rely upon the variation of one or more electrical properties due to the presence of material at different levels within the bin. Thus, capacitive reactance of one or more probes may be affected to signal the amount of material present. The operation of an oscillator circuit, including a resonant probe, may be affected to operate a control system in response to the presence of material near the resonant probe.
U.S. Pat. Nos. 3,807,231 and 3,935,970 disclose a measuring system, and related control system, wherein the reactance of an antenna probe varies as a function of the level of material in the container adjacent the antenna. The antenna reactance varies the frequency of a transmitter signal, which is then combined with a constant frequency reference signal. The frequency of the resulting difference signal is then used to operate a material level indicator, and to provide control information for operating automatic systems for controlling the material level.